Radio-Echo Sounding: Reflections From Internal Layers In Ice Sheets

Abstract

ABSTRACT. The OrIgm and nature of radio-echo in terna l reflections is the subjec t of much debate. Calculations of sin gle-and multipl e-l ayer refl ection coefficients for dielectric changes in the ice sh eet indicate that the observed refl ec tion strength may result from reflec ti ons from many thin layers but the zone of thin layers may bes t be approx imated by re placing the zone with a singl e thi cke r layer with average dielectric properties. Calculation of refle ctions fro m density changes, dust and dirt bands and an isotropy of. indica te d e nsity variations as the most likel y cause of the internal layering in the uppe rmost kilometer of ice sheets.Comparison of the d epth of internal rcfl ec tions at " Byrd" station , Antarctica, with the physical properties of the ice meas ured in a deep core hol e at that loca tion support this conclusion.R ESUME. Sondages par echo radio: R ejfexiolls sur les niveaux illtern es dall s les caloUes glaciaires. L 'origine et la na ture des refl exions internes de I'echo radio sont le suj e t d e nombreux debats. Les calculs de coefficients de reflexion d'un niveau unique ou d e niveaux multiples de changement dielectriques dans la calotte glaciaire montrent que I'intensite des n:flexion s observees peut res ulte r de n:fl ex ions sur un grand nombre d e couches minces mais que la zone de ces cou ch es minces peut etre simulee au mi eux en r e mpla~ant cette zone par un seul niveau plus e pais avec des proprietes di elec triques moyennes. Le calcul des reflexions provenant des ch a ngements d e d e nsite, de lits d e pouss ierc ou de boue e t d'a nisotropie de • indique que les variation~ de d e nsite sont la cause la plus proba ble d e la stra tifica tion interne dans le kilometre superieur des calottes glaciaires. La comparaison de la profondeur des refl exi o ns internes a la Station "Byrd" dans I' Antarctique avec les proprie tes physiques de la g lace mesuree par un caro ttage profond dans les memes parages confirme ces conclusions.ZUSAMM ENFASSUNG. Radar-Echololllllg: R ejfexiollCll an illneren Schichten in Eisdecken. Der Ursprung und Charakter inn eren R efl ex ion en beim R ad a r-Echo wird h a ufig diskutiert. Berechnungen der R eflexionskoeffizi enten fur diel ektrische Schwan kungen in del' Eisd ecke bei einfach e r und mehrfacher Schichtung zeigen, dass die beobachtete Refl ex ionss tarke durch R e Aexione n a n vielen dunnen Schichten erzeugt werden ka nn , dass aber di e Zone dunner Schi chte n am bes ten durch Ersatz der Zone mit einer einzelnen dickeren Schicht von mittl ere n dielektrischen E igenschaften anzunahern ist. Die Berechnung der R e Aexion an Dichtesprungen , Sta ub-und Schmutzban dern und di e Aniso tropic von • e rweisen Dichteschwankungen als die wa hrscheinli chste Ursache fur die innere Schi chtun g in dem obersten Kilom eter von Eisd ecke n. Ein Ve rgl eich zwischen d el' Tiefe inn erer R e Aex ionen a n del' "Byrd" Sta tion, Antarktika, mit den physikalischen Eigenschaften d es Eises, gemessen in e inem tiefen Bohrloch an diesel' ...

Superimposed ice regime of a high Arctic glacier inferred using ground‐penetrating radar, flow modeling, and ice cores

“…Hence the reflecting horizons in zone II are interpreted to arise primarily from alternating bubble-poor and bubble-rich layers in superimposed ice, which give rise to small variations in ice density. The production of IRHs by alternating clear and bubbly layers in solid ice has been has been reported in other studies of polar ice masses [Robin et al, 1969;Clough, 1977]. The superimposed ice layers documented here are isochronous, formed by refreezing of snowmelt at different stages of a summer melt season.…”

Modeling the radio echo reflections inside the ice sheet at Summit, Greenland

“…[9] Deep inside ice sheets, densification reduces the variation in e 0 r1 and variation of other properties becomes the dominant mechanism causing reflections. The calculated depth at which density variation is no longer dominant has become shallower as research has progressed: 1500 m [Paren and Robin, 1975], 1000 m [Clough, 1977;Robin et al, 1969], 500 m [Millar, 1981b], 250 m [Moore, 1988a]. Recent work by Fujita et al [1999], measuring at two frequencies in Antarctica, has put the transition just below 1000 m. However, the densification for any particular site will vary with the latitude and local climate.…”

“…If a feature in the ice core can be dated and correlated with a reflection, then both can be given the same date. There have been many comparisons of profiles of ice core properties against radio echo profiles collected at the same site [e.g., Clough, 1977;Ackley and Keliher, 1979;Hammer, 1980;Millar, 1981bMillar, , 1982Nishio and Ohmae, 1985;Yoshida et al, 1987;Blindow, 1994a]. The problems of such comparisons are discussed by Gudmandsen [1975].…”

Nature of radio echo layering in the Antarctic Ice Sheet detected by a two‐frequency experiment

“…Reflections of radio waves from within the ice are caused by sudden changes in complex dielectric permittivity of layers in the ice sheets. Since the first report of internal layers [Bailey et al, 1964], various mechanisms have been suggested and investigated [Ackley and Keliher, 1979;Clough, 1977;Fujita and Mae, 1994;Gudmandsen, 1975;Harrison, 1973 1 and 2). This paper reports the major results to date and will show that by radar sounding we can assess and interpret the internal physical processes and their changes, one of the most important keys to interpreting mass balance.…”